This invention relates generally to impurity addition to crystalline structures and elements, and more particularly, to adding impurities to crystalline structures and elements, such as, but not limited to, natural diamond, synthetic diamond, and the like, by a method which utilizes a combination of temperature, material properties and geometry to achieve contact diffusion of the impurity into the crystalline structure, and products made by the method.
Currently, natural diamond, synthetic diamond and other like crystalline elements, have been used individually, and in aggregate form, as components of slurries, films, composites, and the like, in abrasives, cutting elements, semi-conductors, and other applications. For such applications, diamond and the other crystalline elements are required to have certain mechanical, chemical and electrical properties, the enhancement of which would provide both economic and functional advantage.
As an example, when used as an abrasive, diamond is subject to the stress of grinding and potentially high temperatures. At high temperatures, diamond is subject to graphitization and/or chemical attack from oxygen. Impurities such as silicon and boron in a diamond crystal help the crystal to resist both the graphitization process and chemical attack from oxygen.
It is also important that diamond abrasives have good mechanical properties. Synthetic diamond, which represents a large portion of the industrial diamond market, has inherent defects in the crystal structure due to the growth process. These defects have an adverse effect on the mechanical properties of the material. It has been shown that the mechanical properties of synthetic diamond can be improved by the addition of impurities into the crystal.
Diamond and beryl are examples of crystalline elements wherein the optical properties thereof are important when they are used as gemstones. The optical properties of these crystalline elements can also be improved by the addition of impurities.
Additionally, the semi-conductor industry has long desired to use diamond as a semi-conductor material due to its advantageous temperature resistance and other properties. However, making diamond an N-type semi-conductor has long been a problem.
It is known to make impurity additions to diamond and other crystalline elements by methods such as insitu doping (adding impurities during the growth phase) and by ion implantation. Both processes have significant problems, most important among which in the present context is an unacceptable level of defects in the resulting crystalline structure. Defects tend to weaken the desirable properties of diamond, albeit mechanical, chemical, optical or electrical.
Addition of impurities during the growth phase of a synthetic diamond crystal leads to defects because there is a large amount of free energy available during the growth phase. Any imperfections introduced as the crystals grow, such as impurity defects, tend to be points where bulk defects can take root during the growth phase. This can seriously degrade the quality of the diamond crystals thus produced.
Addition of impurities by implantation also leads to an unacceptable level of defects. In the implantation process, energetic ions are present which are a source of free energy. As the ion streams through the crystal, energy is deposited in the crystal and bulk defects occur around the path of the ion. The damage to the crystal is further enhanced due to a cascading effect that radiates outward from the ion path. Even after annealing the crystal, an unacceptable level of defects remain in the crystal structure.
The known industrial processes such as ion implantation and insitu growth are well described in the literature. See, for example: David Dreifus and Bradley Fox, xe2x80x9cActive Devices,xe2x80x9d in Handbook of Industrial Diamonds and Diamond Films, M. Prelas, G. Popovici, and K. Bigelow editors (Marcell Dekker, 1998) pages 1043-1072. R. Kalish and S. Prawer, xe2x80x9cIon Implantation of Diamond and Diamond Films,xe2x80x9d Handbook of Industrial Diamonds and Diamond Films, M. Prelas, G. Popovici, and K. Bigelow editors (Marcell Dekker, 1998) pages 945-982.
Reference also Popovici et al. U.S. Pat. No. 5,597,762 issued Jan. 28, 1997 entitled Field-Enhanced Diffusion Using Optical Activation, which discloses an ion implantation process for doping diamond material with impurities to make N-type diamond semi-conductors usable in electronic, optical, and other applications. However, the method of the Popovici et al. patent has been found to be limited to use on solid diamond substrates including films and the like, as it is required to create a high voltage across the substrate to provide polarization for ion flow. When the process is attempted to be used with crystalline powders and aggregates, it has been found that the required voltage cannot be achieved, essentially due to shorting of the ion paths.
Additionally, both ion implantation and insitu growth are slow. For instance, it has been found that when the ion implantation method disclosed in Popovici et al. is used, a maximum impurity density in the diamond of only about 0.05 parts per million is achieved after 8 hours of diffusion.
Thus, what is required is a method for the addition of impurities into diamond and other crystalline structures, which involves low free energy so as to avoid defects and resultant crystalline degradation, which can be used with diamond and other crystalline structure in powdered or granular form, and which adds the impurity at a faster rate than known diffusion processes.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, a low free energy method for more rapidly diffusing an impurity into a diamond or other crystalline element in powdered or granular form, without degradation of the crystalline structure, is disclosed. The present method includes the steps of providing a mixture of the diamond or other crystalline element and the impurity in a solid phase; treating the mixture to bring the impurity into substantially conforming contact with substantially all of the outer surface of the crystalline element; and heating the mixture to a temperature between about 200xc2x0 C. and about 2000xc2x0 C.
A feature of the present invention is the capability of diffusing the impurity into a crystalline element such as natural or synthetic diamond, beryl or goshanite in an individual crystal or powdered form having a grain size ranging from as small as a very small fraction of a micrometer to greater than a millimeter.
Another feature of the invention is that the impurity can consist of a wide variety of substances, including, but not limited to, boron, lithium, nitrogen, oxygen, fluorine, sodium, aluminum, sulfur, chlorine and compatible mixtures thereof.
Another feature of the invention is that the impurity can be diffused into the crystalline element in an amount ranging from as little as a fraction of one part of the impurity per 1 million parts of the crystalline element to as much as several thousand parts of the impurity per 1 million parts of the crystalline element or more.
As another feature of the invention, the heating of the mixture to diffuse the impurity into the crystalline element can be for a time period of as little as a fraction of a minute to several hundred hours.
As another aspect of the present invention, a diamond having an impurity diffused into the crystalline structure thereof at a ratio of from about 0.1 part of the impurity per 1 million parts of the diamond to about 600 parts of the impurity per 1 million parts of the diamond, is disclosed.
Another feature of the invention is that the fundamental crystalline structure is virtually unchanged by the diffusion of the impurity therein, such that mechanical, chemical, optical and electrical properties of the diamond crystalline are unchanged and in some instances, are enhanced.
As another feature of the invention, by the diffusion of boron into diamond, the oxidation and graphitization resistance properties of the diamond are enhanced.
As another feature of the invention, by the diffusion of lithium into diamond, the diamond becomes an N-type semi-conductor.